This invention concerns a repeater jamming transmitter being arranged to be ejected out of an aircraft and to operate at a distance from the aircraft in motion, comprising receiver antenna, amplifier and transmitter antenna for receiving, amplifying and transmitting signals, the receiver antenna and the transmitter antenna each comprising at least one antenna pair. The invention also concerns a casing arrangement with repeater jamming transmitter.
Repeater jamming transmitters generate an amplified signal by receiving a radar signal, amplifying it in an amplifier and sending the signal out again. The amplified signal creates a mock target for the radar which the radar perceives as having a greater target area the higher the amplification which can be achieved. It is therefore important to attain a high amplification.
On account of leakage between the transmitter antenna and the receiver antenna part of the power sent out in the transmitter antenna will, however, be received by the receiver antenna and amplified. If the amplification at any frequency exceeds the isolation between the transmitter antenna and the receiver antenna, self-oscillation will arise and the mock target will thereby cease to act in the intended way. The isolation between the transmitter antenna and the receiver antenna can be increased by increasing the distance between the antennas and thereby permitting an increase in the amplification. It is, however, desirable to have a repeater jamming transmitter which has as small dimensions as possible. A mock target is thus created with favourable characteristics. At the same time the repeater jamming transmitter takes up less space and can be made lighter, which makes it particularly suitable for installation as airborne equipment. The compact format can also help to keep down the manufacturing costs, which is of particular value as the repeater jamming transmitter is completely separated from the aircraft after use.
The aim of this invention is to achieve a repeater jamming transmitter with small dimensions which can provide high amplification and has the other benefits of repeater jamming transmitters with small dimensions mentioned above. The aim of the invention is achieved by a repeater jamming transmitter characterized in that the repeater jamming transmitter is designed with broad-band and comprises a connection arrangement for maintaining isolation between the receiver antenna and the transmitter antenna in the angular area where reception and transmission take place simultaneously.
According to one favourable embodiment the connection arrangement for maintaining isolation between the receiver antenna and the transmitter antenna is constituted by an antiphase connection applied to the antenna pair of the transmitter antenna and/or receiver antenna. The transmitter antenna and the receiver antenna can thereby each comprise one or more pairs of antennas and according to a suitable embodiment it is proposed that the transmitter antenna and the receiver antenna comprise two pairs of antennas turned in relation to each other with the aim of receiving and transmitting in two preferably orthogonal polarisations turned in relation to each other.
The pairs of antennas of the receiver antenna and the transmitter antenna can comprise a first and a second outer conductor designed with antenna ribs, and a central conductor. According to a preferred embodiment in order to achieve an antiphase connection in a pair of antennas one antenna tip is connected between the first outer conductor and the central conductor and the second antenna tip is connected between the second outer conductor and the central conductor. According to another preferred embodiment in order to achieve an antiphase connection in a pair of antennas the direction of the antenna ribs in one antenna is arranged in a reversed direction in relation to the second antenna.
The antiphase connection improves the isolation between the transmitter antenna and the receiver antenna. By having one pair of antennas of two corresponding pairs of antennas, one for reception and one for transmission, work in the same phase while the other works in antiphase, the leakage between the transmitter antenna and the receiver antenna will be phased out. The antiphase connection is carried out in connection with the input of the amplifier. However, this places large demands on accuracy in installation as electrical wavelengths from both antennas in the pair of antennas must be very accurately matched. Temperature increases can cause the wavelengths to change, with reduced isolation as a result. By making the antiphase connection direct at the antennas the wavelength problem is avoided. This solution is also cheaper than making the antiphase connection in association with the amplifier.
If one pair of antennas in the same phase and one in antiphase are arranged in the same plane the pair of antennas in antiphase creates a null depth in the normal direction of the pairs of antennas. This null depth coincides with the beam maximum of the pair of antennas connected in the same phase. As the target area is the product of the antenna gain, the value of this beam maximum will decrease. According to a preferred further development of the repeater jamming transmitter it is therefore proposed according to the invention that the antenna planes of the receiver antennas are to be turned in relation to the antenna planes of the transmitter antennas. In particular it is proposed that the turning between the antenna planes of the receiver antennas and the antenna planes of the transmitter antennas is to be of the order of 20xc2x0. By turning one antenna plane in relation to the other in this way the null depth can be moved to a direction where it does less harm. By means of the turning the average target area is increased defined over a conical cross section, which is valuable in many fields of application associated with repeater jamming transmitters.
When the amplification is maximized with regard to the isolation, it must be taken into account that the amplification varies with the temperature. This means that there must be margins for coping with the worst cases. In this way considerably poorer performance is obtained also in other cases than what it is possible to achieve without self-oscillation arising in that specific case. A considerable increase in performance can be achieved for the majority of cases by introducing an automatic amplification regulation so that the highest possible amplification is set at different operating instances by measuring when self-oscillation arises in that specific case and thereafter reducing the amplification slightly so that the self-oscillation ceases. According to a further preferred embodiment which permits an increase of the amplification, the repeater jamming transmitter is characterized in that the connection arrangement comprises a means of automatic regulation of the amplification by detection of the amplification level for self-oscillation and reduction of the amplification to a level slightly below the amplification level for self-oscillation.
Advantageously, the means for automatic regulation can comprise an amplification control, power meter and switch arranged between the receiver antenna and the transmitter antenna, whereby reduced power with the switch open in comparison to with the switch closed indicates self-oscillation.
The repeater jamming transmitter can be housed in a casing together with a cable brake attached to the bottom part of the casing. This casing arrangement has obvious advantages as far as handling is concerned.